1. Field of the Invention
The present invention belongs to a mechanical field relating to an objective lens and a recording/reproducing apparatus for recording/reproducing an information signal to an optical recording medium such as an optical disc having the objective lens.
2. Description of the Related Art
Conventionally, an optical recording medium such as an optical disc is widely used due to mass-production and low cost for reserving data such as moving picture information, voice information, data for a computer. Recently, high density and large capacity is requested more and more because an information-oriented society is sharply progressing.
In order to improve recording density of the optical recording medium, both of a short wavelength of a flux of light for reading out an information signal and a high numeral aperture of an objective lens (using the objective lens having the high numeral aperture) for collecting the flux of light on the respective optical recording medium are effective.
The NA of the objective lens for so-called "a compact disc (CD)" (a digital optical disc for an audio signal or computer data) is 0.45, on one hand, the NA of the objective lens for so-called "a digital video disc (DVD)" (a digital optical disc for a video signal) the recording density of which is improved rather than "the compact disc" is 0.6.
The objective lens for these optical disc is formed as a non spherical single lens (a single ball non spherical lens) made of synthetic resin and glass materials.
"The digital video disc (DVD)" has a disc substrate which is 0.6 mm thick that is a half of the thickness of the disc substrate of "the compact disc" and an optical magnetic disc in order to reduce influence of comatic aberration due to a gradient of the disc.
In the oon spherical single lens having the numeral aperture (NA) higher than that used at present.
One of reasons is processing of metallic mold. That is to say, when the gradient of the surface of the lens is larger than 45 degrees as reference is vertical to an optical axis, it is hard to process the non spherical metallic mold due to a size of an end of a diamond cutting tool. Further, the sharper curvature is, the larger depth (called "sag") in the direction of the optical axis from the top of the lens to the outest circumference is, therefore, the cutting processing of the metallic mold is hard.
If the processing is possible, in an area that the numeral aperture (NA) is rather than 0.7 as an single lens, high degrees aberration correction higher than 7 degrees is essential. Further more free degrees of design are necessary for it. Though it is considered to make use of a higher degrees non spherical coefficient for correction it, it is hard to process and design, therefore, it is not an effective means.
Under these circumferences, though the non spherical lens the numeral aperture (NA) of which is more than 0.8 is reported as the design, there is no report that the lens the numeral aperture of which is larger than 0.7 is made as a single lens.
On the other hand, the lens having the high numeral aperture that the numeral aperture is larger than 0.7 used for a conventional microscope and so on is a combination lens including plural of spherical lenses for which plural kinds of glass materials are used. From an optical similarity between the microscope and an optical pick-up apparatus for the optical disc, it is easily considered to apply the same lens as that for the microscope for the optical lens of the respective pick-up lens. Further, at the dawn to make the optical lens practicable, the combination lenses including two groups of three spherical lenses were used.
However, these combination lenses are necessary to be precisely assembled, and unstable performance and high cost is inevitable.
This problem has been solved by the non spherical mold single lens. In the non spherical mold lens, not only it is possible to obtain the non spherical form having high precision, but also it is not necessary to assemble due to a single ball and it is possible to obtain sufficient tolerance to decentering and falling. However, in the non spherical mold single lens, as above mentioned, it is difficult to make the non spherical single lens having the numerical aperture (NA) higher than that used at present.